1. Field of the Invention
The present invention relates to an electrophotographic toner. More specifically, the invention relates to a black toner for black and white printing and an electrophotographic toner for full color image formation used in an electrophotographic image forming apparatus such as an electrostatic copier or a laser printer.
2. Description of the Related Art
In recent years, an electrophotographic black-and-white forming apparatus has been used widely. Images are formed in an electrophotographic black-and-white forming apparatus by the processes of charging, exposure, development, transfer and fixation. First, in the charging process, the surface of an insulating photoreceptor is uniformly charged. In the exposure process, the surface of the photoreceptor is irradiated with light corresponding to image information so that a latent electrostatic image is formed thereon. In the development process, a black toner is adhered selectively to this latent electrostatic image so that a toner image is formed on the surface of the photoreceptor. Then, in the transfer process, the toner image is transferred onto a transfer sheet by electric force. In the last process of fixation, the toner image transferred onto the transfer sheet is melted by heat so as to be fixed on the transfer sheet.
In this black and white forming apparatus, low energy (reduction in power consumption), high-speed of processing and high performance are required, as this apparatus is increasingly widely spread. With respect to these requirements, it has been proposed to achieve low energy, and high-speed, simplification and stabilization of processing for a fixation device for fixing a toner image in a transfer sheet. In general, as the fixation device, those employing heating rolls are used. The fixation device employing heating rolls includes a fixation roller having a built-in heater and a pressure roller. A transfer sheet is passed between the fixation roller and the pressure roller, and heat and pressure are applied to the transfer sheet so that a toner image is melted and fixed.
In such a fixation device, a black toner having a property of low temperature fixation that allows satisfactory fixation even at a low temperature and a wide fixation temperature region have been under development for low energy, and high-speed, simplification and stabilization of processing. Among these, regarding the property of low temperature fixation, there is an attempt to use a low molecular weight binding resin. In order to achieve a wide fixation temperature region, there is an attempt to use a wax that is melted out at a low temperature and prevents a toner from adhering to the fixation roller by covering the surface of the fixation roller.
However, when an image is formed by a black toner containing a low molecular weight binding resin, the high temperature offset phenomenon in which a toner adheres to a fixation roller during heat melting tends to occur readily. This is because in such a toner, cohesion between toner particles is reduced so that the toner layers are separated. Therefore, there is a demand for development of a good black toner that hardly allows the high temperature offset phenomenon.
Recently, the multicolor electrophotographic technique has been developed rapidly, and thus full color image forming apparatuses have been developed. The market of the full color image forming apparatuses has been enlarged with the wide spread of black and white image forming apparatuses. In general, for reproduction of color in the full color image forming apparatuses, a color toner having three colors of yellow (Y), magenta (M), cyan (C), which are the three primary colors of subtractive mixture of colors, or a color toner having four colors of these three colors and black (K) is used. For procedure of reproduction of colors, with respect to each color of C, M, Y and K, the processes of charging, exposure, development and transfer of the image formation processes are repeated so that toner images formed of color toners of a plurality of colors are superimposed and formed on a transfer sheet. Then, in the last process of fixation, the superimposed toner images are melted so as to be fixed on the transfer sheet. By this procedure, the superimposed toner images are mixed by being melted, so that colors are reproduced based on the principle of the subtractive mixture of colors.
Thus, in the full color image forming apparatus, the processes from charging to transfer for each color are repeated, and toner images formed of color toners of a plurality of colors are superimposed on a transfer sheet, and therefore the melting characteristics of color toners with respect to the transfer sheet are very important. That is to say, the color toners should be melted to an extent that the boundaries between the toner particles are eliminated so that color reproduction is not prevented by diffused reflection of light by the fixed toner particles and should have a wide color reproduction range in which transparency can be provided. In addition, appropriate gloss and luster are required.
For the full color image forming apparatus, in a fixation device for fixing a toner image made of a color toner, a fixation roller made of a material having good surface releasing properties is used, and a large amount of oil is applied to the surface of the fixation roller in order to improve the releasing properties further. In general, a large amount of oil is applied to the fixation roller for the following reasons.
As described above, the melting characteristics of a color toner are important, so that the color toner is formed of a material having a lower viscosity and a larger thermal melting properties than those of a black toner for a black and white image forming apparatus. However, when a binding resin having a low viscosity is used for a toner, the cohesion of the toner is poor, so that the toner layers are separated, and the offset phenomenon in which the toner partially adheres to the fixation roller tends to occur readily, which results in non-uniformity in an image. In order to reduce the non-uniformity in an image, a large amount of oil is applied to the fixation roller.
However, when a large amount of oil is applied to the fixation roller, a transfer sheet may be soiled with oil, and the cost may increase. Furthermore, a space in which oil is stored is required, so that another problem is caused in that the structure of the color image forming apparatus becomes complicated or large.
In order to solve the above problems, a so-called “oil-less toner” is proposed. An “Oil-less toner” refer to a toner in which the occurrence of the high temperature offset phenomenon is reduced by a certain amount of wax that oozes from the toner particles and penetrates between the toner and the fixation roller during melting for fixation. With this oil-less toner, it is attempted to put into practice an oil-less fixation device that does not apply oil to the fixation roller, and thus the problems of soiling a transfer sheet with oil, increasing the cost and resulting in a complicated and large structure are being solved.
However, it is for a black and white image forming apparatus that it is attempted to put the oil-less fixation device into practice, and there is a problem in using the oil-less fixation device for a full color image forming apparatus. This is because when fixing color toners having a lower toner cohesion than that of the black toner with the oil-less fixation device, the offset phenomenon occurs and thus non-uniformity in the images occurs. Therefore, in order to fix color toners with the oil-less fixation device, it is necessary to adjust the amount of the wax that oozes out so that the effect of oozing the wax is sufficiently exhibited.
The amount of the wax that oozes from the toner depends on the compatibility between the binding resin and the wax contained in the toner. When the compatibility is too good, the binding resin and the wax are integrated, which makes it difficult that the wax oozes out. On the other hand, when the compatibility is poor, the wax is dispersed poorly in the toner particles so that the wax is present in the form of a large lump non-uniformly in the toner particles or on the surface thereof. This may deteriorate the charging characteristics, which are the characteristics of the toner, or generate filming to the photoreceptor or the carrier. Furthermore, during melting for fixation, because of this large lump, low viscous wax layers are generated in the toner and separation occurs in the toner layer, and thus the high temperature offset phenomenon tends to occur easily.
The techniques for improving the fixation properties of a toner are disclosed in Japanese Unexamined Patent Publications JP-A 3-50560 (1991), JP-A 8-106173 (1996), JP-A 9-179342 (1997) and JP-A 2000-338707 (2000).
JP-A 3-50560 discloses a developing toner for an electrostatically charged image containing a paraffin wax that is modified with carboxylic acid or an acid anhydride thereof.
JP-A 8-106173 discloses a developing agent (toner) containing a binder resin (binding resin) having an acid value of 5 to 50 mgKOH/g made of a polyester resin or a mixture of a polyester resin and styrene-acrylic resin and two kinds of waxes having different melting points.
JP-A 9-179342 discloses an electrophotographic toner containing a natural gas7based Fischer-Tropsch wax having an acid value of 1 to 5 mgKOH/g and a polyester resin having an acid value of 1 to 5 mgKOH/g.
JP-A 2000-338707 discloses a developing toner for an electrostatically charged image containing an alcohol-based paraffin wax including a saturated hydrocarbon having 20 to 60 carbon atoms, having a melting point of 65 to 110° C., an acid value of 5 to 25 mgKOH/g, a saponification value of 20 to 50 mgKOH/g, and a hydroxyl value of 50 to 120 mgKOH/g.
As described above, regarding black toners, it is a challenge to develop a black toner that can satisfy the requirements of low energy, high-speed and high performance to a higher extent for an electrophotographic black-and-white image forming apparatus. Furthermore, regarding color toners, it is a challenge to develop a color toner that can be used in a full-color image forming apparatus provided with an oil-less fixation device.
That is, regarding black toners, it is necessary to develop a black toner having good fixation characteristics such as the property of low temperature fixation and a wide fixation temperature region. Regarding color toners, it is necessary to develop a color toner that has good fixation characteristics, can exhibit a good effect of oozing wax and has high transparency and good gloss properties.
In the developing toner for electrostatically charged images disclosed in JP-A 3-50560, a carboxylic acid-modified paraffin wax having better fixation strength and releasing properties than those of conventionally used polyethylene is used. However, the property of low temperature fixation or the effect of oozing the wax are not considered.
In the toner disclosed in JP-A 8-106173, the compatibility between the binding resin and the wax and the effect of oozing the wax are considered, and high speed fixation is aimed at. However, it is not considered to ensure a wide fixation temperature region.
In the electrophotographic toner disclosed in JP-A 9-179342, it is attempted to improve the property of low temperature fixation, the fixation strength and the compatibility with the binding resin by providing the Fischer-Tropsch wax with a polar group. However, it is not considered to ensure a wide fixation temperature region or the effect of oozing the wax.
In the developing toner for electrostatically charged images disclosed in JP-A 2000-338707, an appropriate compatibility with the binding resin and a wide fixation temperature region are ensured by allowing the toner to contain a predetermined alcohol based wax. However, the effect of oozing the wax is not considered.